The invention relates to a mixing device of an exhaust gas purification system of a motor vehicle internal combustion engine for mixing urea solution with exhaust gas of the motor vehicle internal combustion engine.
To remove nitrogen oxides from the exhaust gas of especially lean operating motor vehicle internal combustion engines by selective catalytic reduction (SCR) on a so-called SCR catalytic converter, it is common to add aqueous urea solution to the exhaust gas. By thermolysis and/or hydrolysis, ammonia is released in the hot exhaust gas from the urea, which acts as the actual reducing agent for the nitrogen oxides and reduces in the catalytic sites of the SCR catalyst the nitrogen oxides selectively to nitrogen. Here there is a difficulty in distributing the urea solution in the exhaust gas as evenly as possible and to avoid deposits. For this purpose, it has been already proposed to apply a rotating flow to the exhaust gas, prior to the addition of the urea solution. For example, from US 2013/0064725 A1, a mixing device of an exhaust gas purification system is known in which exhaust gas from an exhaust gas tube is supplied to a mixing tube which is perpendicular to the former, wherein the exhaust gas is diverted in its main direction of flow and a rotational flow is applied to it. Urea solution is sprayed into the rotational flow by means of metering device at one end of the mixing tube.
It is an object of the invention to provide a mixing device for mixing urea solution in the exhaust gas, wherein the mixing is further improved and deposits can be more effectively avoided.
The mixing device according to the invention comprises a straight circular cylindrical swirl tube having a first end and a second end opposite the first end and an outer surface with a cutout opening. In the swirl tube a circular cylindrical mixing tube is coaxially arranged comprising an open first end disposed inside the swirl tube and which penetrates a bottom portion of the second swirl tube end. Further, an exhaust gas supply tube, which at least approximately tangentially enters the swirl tube is provided in such a way that exhaust gas coming from the exhaust gas supply tube can at least approximately tangentially enter the swirl tube through the cut out opening in the lateral surface of the swirl tube, and after its entry into the swirl tube forms a rotating flow around the mixing tube, wherein it completely enters the open first end of the mixing tube while maintaining a rotating flow and completely leaves the interior of the swirl tube through the mixing tube. By means of a metering device, which is disposed on the first end of the swirl tube, the urea solution can be sprayed with a conical spray pattern into the swirl tube in such a way that it at least almost completely enters the first open end of the mixing tube and is entrained by the rotating flow of the exhaust gas and is discharged, together with the exhaust gas, through the mixing tube, from the interior of the swirl tube. Maintaining a rotating flow does not necessarily mean that an originally existing swirl is maintained with respect to strength and direction of the flow velocity, but that a rotating flow, which has been applied to the exhaust gas when flowing into the swirl tube, and which essentially helically rotates around the mixing tube is continued in the mixing tube, as here the gas flow has an average angular momentum vector, which also has an axially directed component. In the end, due to the inventive design of the mixing device, in the swirl tube, recirculation of exhaust gas and flow dead zone and hence deposits can be largely avoided.
Between the mixing tube open first end and the first end of the swirl tube, a certain back pressure arises from the fact that the exhaust gas is completely forced into the mixing tube, so that the pressurized urea solution which is sprayed from the metering device is additionally accelerated. Since the exhaust gas entering the swirl tube flows around the mixing tube, this is additionally heated by exhaust gas, which effectively prevents the formation of crystallization deposits in the mixing tube. The corresponding acute angle of the cone-shaped spray ensures that urea solution reaches at least approximately only the interior of the mixing tube and thus that it virtually does not wet the inside of the typically somewhat colder swirl tube and then forming deposits. Already in the mixing tube a preparation of the urea solution can take place, not only by mixing with the exhaust gas, but also by evaporation of aqueous components and further through a more or less strong decomposition of urea with the release of gaseous ammonia as a result of thermolysis and/or hydrolysis. In this context, a common discharge of urea solution and exhaust gas from the mixing tube is to be understood in the sense that converted constituents of the urea solution are discharged from the mixing tube together with the exhaust gas. Due to the formation of a rotatory exhaust gas flow throughout the swirl tube the mixing of urea solution with exhaust gas already takes place immediately after the spraying. Due to the existing rotational flow of the exhaust gas also in the mixing tube, a continuous mixing and preparation takes place, and further an even distribution of the sprayed urea solution through the mixing tube cross-section is made possible.
In an embodiment of the invention the mixing tube has a closed lateral surface. This ensures that exhaust gas can only enter the open end opposite the metering device, and that the rotating exhaust gas flow caused by the at least approximately tangential exhaust gas inlet into the swirl tube is maintained until the inlet of the mixing tube and further beyond.
In a further embodiment of the invention, the exhaust gas supply tube at its inflow into the swirl tube has an axially expanding, with respect to the swirl tube, cross-sectional enlargement. By this measure, flow resistance for the exhaust gas is reduced, in particular in the transition from gas supply tube into the swirl tube, thereby reducing pressure losses. Upstream of the cross-sectional enlargement, the cross section of the exhaust gas supply tube is preferably at least partially uniform and round.
In a further embodiment of the invention, the cutout opening extends along its longest dimension with respect to the swirl tube axis from a portion close to the second end of the swirl tube into a portion close to the open first end of the mixing tube. Preferably, the cutout opening extends up to a few millimeters, that is to within less than 10 mm, in particular less than 5 mm from the second end of the swirl tube. On the opposite side, the cutout opening extends, in relation to the axis of the mixing tube or swirl tube preferably up to less than 30 mm, in particular less than 20 mm from the open first end of the mixing tube. The mixing tube portion arranged in the interior of the swirl tube thus has a length which is only slightly longer than the longest axial extension of the cutout opening. Preferably, the length of mixing tube portion inside the swirl tube is about 125% of the longest dimension in the axial direction of the cutout opening. The mixing tube portion arranged in the interior of the swirl tube preferably has a length which is about 60%-70% of the total length of the swirl tube. This results in a particularly compact design.
In another embodiment of the invention, the cross-sectional area of the mixing tube corresponds at least approximately to the smallest cross-sectional area of the exhaust supply tube, wherein, in a further embodiment of the invention, the difference of the cross-sectional area of the swirl tube and the cross-sectional area of the mixing tube is larger, by at least 30 percent, than the smallest cross-sectional area of the exhaust gas supply tube. As a result of this design, restrictions of the exhaust gas flow path downstream of the confluence of the exhaust supply tube in the swirl tube are avoided. Due to the fact that the free area between the mixing tube and swirling tube is larger than the cross-sectional area of the exhaust gas supply tube, during the inflow of the exhaust gas into the swirl tube a sudden enlargement of the flow cross-section is obtained, which in turn causes an acoustic damping. Therefore, the flow noise is particularly low.
In a further embodiment of the invention, a funnel element conically opening in the direction of the open first mixing tube end is provided with a cone axis which is at least approximately collinear to the swirl tube axis and a perforated conical surface into which the metering device can spray the urea solution at least approximately completely. The funnel element takes the conical spray of sprayed urea solution at least approximately completely. The perforation of the conical surface allows an approximately radially directed exhaust access to the interior of the funnel element. An exhaust gas entrance into the funnel element is, however, also made possible in the axial direction by a circular cylindrical opening at the pointed end of the funnel element, through which the spray passes into the interior of the funnel element. This allows the sprayed urea solution to be captured and entrained by the exhaust gas in the axial direction in the direction of the open end of the mixing tube of the exhaust flow. Preferably by constructive means it is ensured that the exhaust gas flow velocity in the area of the funnel element is more than twice as large as at the entry point into the swirl tube. Preferably, it is also significantly greater than the original speed of sprayed urea solution droplets. This allows an effective entrainment of the droplets and their atomized distribution to form a fine spray. The other open end of the funnel element extends up to a few millimeters, preferably about 20 mm from the open end of the mixing tube. The diameter of the larger funnel element opening is preferably about half of the mixing tube diameter. This ensures that sprayed urea solution can be at least almost completely injected into the mixing tube.
In a further embodiment of the invention, an enclosing tube housing the funnel element with a largely perforated lateral surface is provided. The enclosing tube may be circular cylindrical, or conical, or frustoconical. In case of a frustoconical design a cone opening is preferably provided in the direction of the first swirl tube end. Through the enclosing tube a mechanical support of the funnel element is made possible. For this purpose, the enclosing tube is connected at one end with the larger opening of the funnel element, and is connected, with a forced or form fit thereto. The other end of the enclosing tube is preferably connected on the edge to a bottom portion of the first swirl tube end or with a console carrying the metering device with a force or form fit. With this embodiment, a high mechanical stability is assured. The hole area of the perforation of the enclosing tube preferably constitutes at least half of the total lateral enclosing surface. Thus obstruction of the exhaust gas flow is substantially avoided.
In a further embodiment of the invention, a flow baffle with arms which at least approximately open in a V-shape in the direction of the second swirl tube end are provided, wherein the arms are joined along their V-shaped profile line with a form fit to the lateral surface of the mixing tube and along their other opposite V-shaped profile line at least approximately in touching contact with the lateral surface of the swirl tube and a connecting line of the arms is at least approximately oriented in a direction parallel to the longitudinal direction of the exhaust gas supply tube direction. In the longitudinal direction of the flow baffle, the baffle extends approximately from the connection line at the height of the central axis of the exhaust gas supply tube of arms to at least approximately the bottom of the second swirl tube end. Here, the arm closer to the exhaust gas supply tube is preferably at least approximately aligned in the axial direction of the mixing tube or swirl tube. The opening angle of the arms is preferably about 45 degrees. The flow baffle causes a deflection of the flow of exhaust gas introduced in the swirl tube towards the open end of the mixing tube to form a flow spirally rotating about the mixing tube. This particularly applies to flow filaments of gas, which have flowed close to the second swirl tube end almost completely around the mixing tube.
Advantageous embodiments of the invention are illustrated in the drawings and will be described below, wherein like components are identified in the figures by the same reference numerals. The above-mentioned and hereinafter still to be explained features are usable without departing from the scope of the invention not only in the particular combination of features, but also in other combinations or alone.